The invention relates to a sputter-ion pump and particularly to a sputter-ion pump for use with an electron tube having a thoriated tungsten cathode.
Certain power tubes suffer a loss of performance which increases with time of operation. Such tubes contain thoriated tungsten cathodes which normally operate for 10,000 to 20,000 hours of life before the cathodes lose their ability to emit electrons. The thoriated tungsten cathodes employed in these tubes consist of a tungsten wire or bar to which has been added thorium oxide in amounts of one to two percent by weight. In order to make the cathodes electronically active and to have a long life, the cathodes are normally carburized to a depth such that the carburized area is 20 to 40 percent of the cross-sectional area of the cathode. Carburizing is normally carried out as a separate operation during tube construction. The carburizing procedure consists of heating the cathode to a high temperature within the range of 1800.degree.-2600.degree. K. (usually 2600.degree. K.) in a carbonaceous atmosphere. The carbon reacts with the tungsten to form a layer of tungsten carbide on the surface of the cathode. Variations in temperature and heating time are used to control the depth of the carbide layer. While the carbide layer is necessary for electron emission, the depth of the layer must not be too great or the cathode will become too brittle to use.
The carbide layer enhances the ability of the thoriated tungsten to emit electrons by causing a chemical reduction of the thorium oxide to thorium metal. The thorium metal diffuses to the surface the cathode where it forms a monolayer of thorium atoms. The thorium on tungsten electropositive dipole effect which is used to form a low work function cathode surface is well known and has been described in the literature.
Many power tubes with long operating lives have been analyzed to determine the reason for low electron emission at the end of life. It has been determined that the end of useful electron emission usually occurs when the carbon is depleted from the surface of the thoriated tungsten cathode so that the cathode can no longer reduce thorium oxide to thorium for use in the emission process. In each case a large amount of thorium oxide remains in the base metal of the cathode; however the thorium oxide is unavailable for electron emission because of the lack of carbon on the surface of the cathode.
It is well known in the art to permanently install or couple a getter pump to vacuum tubes such a klystron and image intensifier tubes to maintain a vacuum within the tube. Such arrangements are described in U.S. Pat. No. 3,244,933 to Schmidt et al. issued on Apr. 5, 1966 and entitled, "Device of the Kind Comprising a High-Power Klystron With Getter-ion Pump Connected Thereto", and in U.S. Pat. No. 3,780,501 to Della Porta et al. issued on Dec. 25, 1973 and entitled, "Getter-Pumps". To activate the aforementioned pump and to cause it to getter residual gases and thereby increase the life of the vacuum tubes using such pumps, a supply of getter material such as zirconium, titanium, tantalum or niobium is activated in a known manner to create a gettering surface.
While getter pumps and particularly sputter-ion pumps such as those described in the above mentioned Schmidt et al. patent are useful for obtaining low operating pressure by gettering residual gases generated within the electron tubes, such pumps are incapable of rejuvenating thoriated tungsten cathodes so as to increase useful tube life beyond 10,000 to 20,000 hours.
U.S. Pat. No. 3,542,488 to Hall, issued on Nov. 24, 1970 describes an improved sputter-ion pump having cathode members constructed from at least two different elements in order to increase the pumping capability of the sputter-ion pump. The Hall patent lists forty four different elements, including carbon that may be combined to provide a higher pumping rate. There is no suggestion in the Hall patent that a sputter-ion pump may be used to rejuvenate the thoriated tungsten cathode of an electron tube attached thereto.